1. Field of the Invention
The present invention relates to a clutch device used in an automatic transmission for an automobile.
2. Description of the Related Art
Clutch devices are disclosed in Japanese Patent Application Nos. 6-48806 and 6-69632 (corresponding to Unexamined Patent Publication (Kokai) Nos. 7-259885 and 7-279992) filed in the name of the assignee of the present application, wherein a cam mechanism is provided between two members rotating relatively to each other so that a force generated due to the relative rotation of the members and acting in the circumferential direction is converted to a thrust acting in the axial direction which is used as a clutch-engagement force.
According to the above clutch devices, it is possible to miniaturize the clutch device as a whole by the reduction of the number of clutch disks and by the elimination of a one-way clutch due to the utilization of a booster function and a unidirectional action of the cam mechanism.
It was noticed that a self-tightening engagement which increases the engagement force corresponding to increase of rotational torque can be obtained when a cam angle .theta. is chosen to satisfy .mu.&gt;tan .theta., and a boosting engagement which increases the engagement force corresponding to increase of pushing force which pushes the cam mechanism to one of the relatively rotating members can be obtained when the cam angle .theta. is chosen to satisfy .mu.&lt;tan .theta., where .mu. is a friction coefficient between the clutch disk and the cam member.
On the basis of the above described observation, it is proposed to install two cam mechanisms, one for obtaining the self-tightening engagement and one for obtaining the boosting engagement, i.e. to install a self-tightening cam and a boosting cam in one clutch device, and thereby obtain a self-tightening action when the members relatively rotates in one direction and obtain a boosting action when the members relatively rotates in a direction reverse to the one direction.
Hereafter, the relative rotation of the member by which the self-tightening cam generates the thrust force for the self-tightening engagement is called the normal rotation, or rotation in the normal direction. On the other hand, the relative rotation of the member by which the boosting cam generates the thrust force for the boosting engagement is called the reverse rotation, or rotation in the reverse direction.
However, the above described clutch device generates unnecessary thrust force which causes a frictional loss when releasing the clutch device rotating in the reverse direction, unless the clutch disk and the cam member are separated from each other in an instant.
Therefore, a return spring is attached to give a biasing force for separating the clutch disk and the cam member from each other.
However, the biasing force of the return spring decreases the thrust force generated by the self-tightening cam.
Accordingly, the clutch device will slip before the direction of the torque is reversed when the torque is relieved from the engaging state at the rotation in the normal direction. The slip of the clutch device causes poor driveability, such as a sudden increase of engine speed, due to a sudden decrease of engine load.